We report the source size distribution , as measured by ALMA millimetric continuum imaging , of a sample of 13 AzTEC-selected submillimeter galaxies ( SMGs ) at z _ { phot } \sim 3 –6 .
Their infrared luminosities and star-formation rates ( SFR ) are L _ { IR } \sim 2– 6 \times 10 ^ { 12 } L _ { \odot } and \sim 200 –600 M _ { \odot } yr ^ { -1 } , respectively .
The sizes of these SMGs range from 0 ^ { \prime \prime } .10 to 0 ^ { \prime \prime } .38 , with a median of 0 ^ { \prime \prime } .20 ^ { +0 ^ { \prime \prime } .03 } _ { -0 ^ { \prime \prime } .05 } ( FWHM ) , corresponding to a median circularized effective radius ( R _ { c,e } ) of 0.67 ^ { +0.13 } _ { -0.14 } kpc , comparable to the typical size of the stellar component measured in compact quiescent galaxies at z \sim 2 ( cQGs ) — R _ { e } \sim 1 kpc .
The median surface SFR density of our SMGs is 100 ^ { +42 } _ { -26 } M _ { \odot } yr ^ { -1 } kpc ^ { -2 } , comparable to that seen in local merger-driven ( U ) LIRGs rather than in extended disk galaxies at low and high redshifts .
The discovery of compact starbursts in z \gtrsim 3 SMGs strongly supports a massive galaxy formation scenario wherein z \sim 3 –6 SMGs evolve into the compact stellar components of z \sim 2 cQGs .
These cQGs are then thought to evolve into the most massive ellipticals in the local Universe , mostly via dry mergers .
Our results thus suggest that z \gtrsim 3 SMGs are the likely progenitors of massive local ellipticals , via cQGs , meaning that we can now trace the evolutionary path of the most massive galaxies over a period encompassing \sim 90 % of the age of the Universe .